Materials not applied after importing .obj (ShapeNet)
This problem with the materials in Blender with the ShapeNet objects seems to be a common one. This worked for me (I used ShapeNetSem):
You can convert your .obj models to .gltf format using the obj2gltf package. Be sure that the ShapeNet textures can be found during converting: I had to put all the .jpg texture images inside the same directory as the .obj and corresponding .mtl files (you can see which textures are needed for a model by opening its .mtl file in a text editor). Then import your .gltf models in Blender. They should now load properly.
On top of that, you can also try to turn off Backface Culling and Auto Smooth, in Blender. This wasn't necessary in my case, but I've read it helps for some people.